According to a conventional electrophotographic method, a lithographic printing plate is produced by subjecting a plate for an electrophotographic lithographic printing, which comprises a water-resistant support having formed thereon a layer comprising zinc oxide and a binder, to corona discharge, imagewise exposure, toner development, and fixation, followed by etching treatment.
In general, production of the plate is carried out using a plate-making method in which a voltage is applied to both surfaces of an original plate as shown in FIG. 6. In FIG. 6, exposure light emitted from a light source is condensed by a lens 18. The condensed exposure light forms an image on a master 1' (lithographic printing original plate) which is fed from a feeder 11 with a transport means and has arrived at an exposure section 20 between guide rollers 15 and 16, thereby effecting imagewise exposure to the master 1'. Before the master 1' is transported to the exposure section 20, the master 1' is negatively charged on an upper surface of a photoconductive layer 5 by a negative corona discharge means 12 and also positively charged on a lower surface of the photoconductive layer 5 by a positive corona discharge means 19. Upon imagewise exposure at the exposure section 20, charges on an exposed area disappear by conduction through the photoconductive layer, and charges remain on an unexposed area alone to form an electrostatic latent image. The exposed master 1' is transported to a development-fixation section 17 by a transport means, and therein the electrostatic latent image is developed by adhering toner to the unexposed area and then fixed. Thereafter, the master is subjected to an oil-desensitizing treatment, and then dried to produce a lithographic printing plate.
In the above production method, however, a charging time is required for migration of charges through a support, and thereby a transport speed is controlled and non-uniform charging causes, making it difficult to form a uniform image.
Examples of the water-resistant support which can be used are a paper having been subjected to water-resistant treatment, a metal foil or a composite thereof. In general, the electrophotographic method requires some discharge treatment of a plate in conducting corona charging of a photosensitive layer. Where plate-making is conducted by the method as shown in FIG. 6, charging is achieved by bringing the whole support to have conductivity and also conducting positive corona discharge to the support from the back surface thereof.
When the support used is a paper, conductivity is imparted to the paper by coating it with a solution containing the so-called conductive agent, such as an inorganic electrolyte, e.g., sodium chloride, potassium chloride or calcium chloride, an organic high molecular electrolyte, e.g., a quaternary ammonium salt, or by immersing it in a solution containing such a conductive agent. In this case, the paper can acquire a volume electric resistance of about 1.times.10.sup.9 .OMEGA..multidot.cm.
When the thus treated paper is used as a substrate for a lithographic printing original plate, it is unavoidable, even when the paper has been subjected to water resistant treatment, that a dampening water applied to the printing plate during printing causes partial elongation of the paper on rollers used in printing, namely a plate elongation. As a result, various problems may occur during printing, such as wrinkles forming on the back edge or register changes caused by slipping of the printing plate.
With the intention of protecting a paper support from the influence of water, there have been attempts to coat one surface or both surfaces of a paper support with a conductive filler-containing water-resistant epoxy resin, ethylene-acrylic acid-copolymer, or the like, as described in, e.g., JP-A-50-138904, JP-A-55-105580 and JP-A-59-68753 (The term "JP-A" as used herein means an "unexamined published Japanese patent application"), or attempts to use a conductive resin-laminated paper, such as a paper provided with a conductive filler-containing polyethylene laminate, described in, e.g., JP-A-58-57994 and JP-A-59-64395. In these cases, the paper used is also rendered conductive.
An example of the structure of a lithographic printing plate using such a laminate paper is shown in FIG. 7. The lithographic printing plate 1' shown in FIG. 7 is constituted of a support 2 which is prepared by subjecting a paper to a conductive treatment, conductive layers (laminate layers) 3a and 3b provided respectively on the back and front surfaces of the support, and a photoconductive layer 5 provided on the conductive layer 3b. On the photoconductive layer 5, a toner image 6 is formed.
This conductive resin-laminated paper has a structure that a resin film is provided on one surface or both surfaces of a paper, and the resin film is required to contain a conductive filler. Accordingly, the production cost of such a support is high, and it causes a rise in cost of the lithographic printing plate. Where a paper support is coated with a filler-dispersed resin so as to bring the support to have water resistance, it is unsuccessful to ensure satisfactory physical properties in the coating of resin. Thus, it was difficult to achieve high water resistance and high conductivity at the same time.
In addition, there have been attempts to use a paper to which a metal foil, such as an aluminum, zinc or copper foil, is adhered (hereinafter referred to as "a metal foil-laminated paper") as described in, e.g., JP-B-38-17249, JP-B-41-2426 and JP-B-41-12432 (The term "JP-B" as used herein means an "examined published Japanese patent publication"). In these attempts, the paper to be laminated with a metal foil is also a paper soaked with the above-described conductive agent. The use of such a metal foil-laminated paper can produce improvements in elongation at wetting and tensile strength, so that it can ensure high dimensional stability in the printing original plate. As for such a metal foil-laminated paper, it has been attempted to arrange a metal foil on the back surface, on both surfaces or in the center of the paper. In any of such cases, a lithographic printing original plate having excellent dimensional stability can be obtained. However, in any case, a metal foil must be adhered to one or both surfaces of a paper, resulting in increasing production cost of the support as compared with the laminate paper.